Addition Of A Value To A Null Value

i have the idea that 23+null=null

now following is my case with 2 columns as:

salary bonus
1 null
2 5
3 null

i have to find the total salary(i.e. salary+bonus)

this is how i managed to do it:

select empname,(salary + commission) as 'total salary'
from emp
where commission is not null
union
select empname,salary from emp
where commission is null

now my question is,wht if the case now is different as below:

salary bonus hra
1 null 6
2 5 null
3 null null

now how do i calculate total salary(i.e. salary+bonus+hra)

am a beginner
help appreciated

cheers